In distributed fiber optic sensing, an unmodified fiber optic waveguide is used as a sensor. There are many ways to interrogate a distributed fiber optic sensor, but all of these methods require sending optical energy into the fiber to produce a backscattering of the light which is used to measure a physical property in proximity to the fiber, such as, temperature, vibration, static or dynamic strain, chemical concentration, or pressure. Examples of commercially established methods include distributed temperature sensing (“DTS”), distributed acoustic sensing (“DAS”), and distributed strain sensing (“DSS”).
In such systems, it is desirable to spatially divide the fiber optic cable, which may be many kilometers long, into discrete sensing regions so that the cable is transformed into a dense array of sensors. These sensors are not placed in or attached to the fiber, but instead are created by the way the fiber is interrogated. Ideally, the dense array of sensors will have spacing between the sensors down to the level of a few meters or less if possible, so as to achieve a very fine spatial resolution. This is of particular importance when sensing along the length of a wellbore, where features of interest may be very localized and in close proximity to areas with different properties. For example, wellbore features like perforation clusters, packers, and production zones may need to be spatially separated in any effective measurement.
In distributed sensing, when a light interrogation signal is sent into the fiber, as the light is travelling down the fiber, a continuous backscatter signal is generated. The backscatter signal of interest may consist of one or a combination of Rayleigh, Brillouin, or Raman backscatter. In order to provide an array of sensors spaced closely along the fiber, a method of multiplexing, or dividing up the backscatter response from these sensing regions must be utilized. Well known methods for optical spatial multiplexing include time domain multiplexing, frequency domain multiplexing, and code-division multiplexing.
However, the data storage and processing requirements of conventional systems are disadvantageous. In order to achieve a desired spatial resolution, the optical receiver must detect and sample the backscattered signal at a high speed. As a result, high speed and bandwidth system components are necessary. However, as the bandwidth increases, the performance of the receiver will be degraded by a proportional amount, thereby adversely affecting the integrity of the sensed parameters.